Polymerization of olefins is usually carried out in the presence of a catalyst system containing a Ziegler-Natta type procatalyst carrying internal electron donor(s), an organo aluminium cocatalyst and an external electron donor. Depending upon the kind of internal electron donor used, the polymerization catalyst systems could either be (a) a monoester based catalyst system or (b) a diester based catalyst system.
Monoester based catalyst systems use monoester of aromatic carboxylic acids as internal electron donors. When a monoester based catalyst system is used, the polypropylene productivity is usually low to moderate and polymers having relatively high polydispersity are formed. However, the extent of kinetic control achieved for the polymerization reaction, using such catalyst systems, is high, thus avoiding run away reactions leading to chunk formation. In most cases, monoester based catalyst systems provide polymerization reactions that are self extinguishing in nature i.e polymerization activity reduces with increasing temperature of the reaction.
Diester based catalyst systems typically use diester of aromatic carboxylic acids as internal electron donor. Polymer productivity using such catalyst systems is relatively high. When diester based catalyst system is used, olefins are usually formed with low to medium polydispersity. It is, however, difficult to achieve adequate kinetic control of the polymerization reaction using such catalyst systems.
Catalyst systems that provide combined advantages of monoester based catalyst systems as well as diester based catalyst systems are highly desired. Use of mixture of external electron donors is known to be advantageous in increasing the productivity of polymerization reactions in monoester based catalyst systems and in controlling the kinetics of polymerization reactions in diester based catalyst systems. US 2007/0032376 discloses a monoester based catalyst system comprising an external donor mixture of paraethoxy ethyl benzoate and a silane compound. The catalyst system comprising the external donor mixture show improved activity as compared to the catalyst system containing paraethoxy ethyl benzoate alone as the external electron donor. However, polydispersity of polypropylene synthesized using the monoester based catalyst system need to be lowered further.
US 2007/0032375 disclose diester based catalyst system wherein a external donor mixture of a silane compound and aromatic carboxylic acid ester is used. Better control is achieved in reaction kinetics of the polymerization reaction by the use of the mixture of external electron donors, as compared to the control achieved by the use of a catalyst system having silane alone as the external electron donor. However, there is scope for improvement in the kinetic control achieved using the diester based catalyst systems.